LESSON 25.] AND LKAVK3. 153 



Tlicy serve not only to strengthen the leaf, hut also to hring; in tlie 

 aseeniliiij^ sap, and to distribute it by the veinlets tlirougliout every 

 j)art. The eelhihir jiorlion is the "jreen |)uli), and is nearly the same 

 as the <!;reen layer of the l)ark. So that the leaf may properly 

 enough be reganh-d as a sort of expansion of the fibrous and green 

 layers of the bark. It has of course no corky layer ; but the whole 

 is covered by a trans[)arent skin or epiderinis, resembling that of 

 the stem. 



4u8. The green pulp consists of cells of various forms, usually 

 loosely arranged, so as to leave many irregular S[)aces, or air-pas^ 

 sages, communicating with each other throughout the whole interior 

 of the leaf (Fig. 35()). The green color is owing to a peculiar 

 green matter lying loose in tlu; cells, in form of minute grains, 

 named Clilorojjliyll (i. e. tiie green of '-^r^'r^—-f—^^ — r^^=r7=: 

 leaves). It is this substance, seen '~l\ ' 



through tlie transparent walls of the L 

 cells where it is accumulated, which 2>: 

 gives the common green hue to vege- j, 

 tation, and especially to foliage. |^_ "feL- 



430. The green pulp in most leaves ^ " - ^-'^N. 



forms two principal layers; an upper a- ^ ~ ^ 



one, facing the sky, and an under one, ;pc5c^^j^ 

 facing the ground. The upper one is s'jS 



always deeper green in color than the lower. This is partly owing, 

 perhaps, to a greater amount of chloropliyll in the upper cells, but 

 mainly to the more compact arrangement of these cells. As is seen 

 in Fig. 35 G and 357, the cells of the upper side are oblong or cylin- 

 drical, and stand endwise to the surface of the leaf, usually close to- 

 grilicr, leaving hardly any vacant spaces. Those of the lower part 

 of the leaf are apt to be irregular in shape, most of them with their 

 longer diameter parallel to the face of the leaf, and are very loosely 

 arranged, leaving many and wide air-chambers. The green color 

 underneath is therefore diluted and paler. 



440. In many plants which grow where they are subject to 

 drought, and which Inild theii- lea\(s (hiring the dry season (the 

 Oleander for example), the greater part of the thickness of the leaf 

 consists of layers of long cells, placed endwise and very nuich com- 



FIO. a.'ifi. Prrlion tliniiich tlio tliickiips's of n Icnfiif llip Star Aiii^e (IlliriiinO, of Fli-rida, 

 in.iKiiiru-il. 'I'lic iiiipiT and Ilio lnwrr liiyera of iliirlt-uallcd .iiid niipty cells ro|ircspiit (ho 

 riiidrriiiiH nr skin. All those butwucu are colU ot tlio grevii pulp, containing graind of 

 clilurupliyll. 



